1. Field of the Invention
This invention relates generally to voltage regulators, and more specifically to voltage regulators in an integrated circuit used for supplying voltage to load circuits in the integrated circuit.
2. Related Art
In an integrated circuit (IC) fabricated using a dual-oxide, complementary metal oxide semiconductor (CMOS) process, there is at least one circuit whose components are built with thick oxide devices and at least one circuit whose components are built with thin oxide devices. The thin oxide devices consume less power than do the thick oxide devices; however, the thin oxide devices cannot withstand as high of a voltage without damage as can the thick oxide devices. An IC typically includes at least one voltage regulator. The regulator is built with thick oxide devices because the regulator is biased off the highest voltage available, which is typically the voltage of a battery that supplies power to the IC. The voltage of the battery is typically 1.7 v to 3.6 v, depending on the process used for fabricating the IC. The regulator provides a lower voltage power supply to circuits that are constructed with thin oxide devices, and a higher voltage power supply to circuits that are constructed with thick oxide devices. Depending on the process for manufacturing the IC, the regulated level of the voltage at the output of the regulator is typically 0.9 v to 1.6 v for thin oxide devices. Any occasion that the voltage at the output of the regulator goes above the regulated level, the thin oxide devices of the IC are at risk of being damaged, possibly leading to a circuit failure. The output of the regulator going above the regulated level is more likely to occur during connection of the battery to the IC (“battery insertion”), which is when the voltage supply increases abruptly from zero volts.
FIG. 1 illustrates a known circuit 100 that includes an IC 101 within an IC package 102. The IC package 102 has a plurality of pins 103 for connecting the IC 101 to circuits external to the IC package 102. The IC 101 includes a thick oxide portion 104 and a thin oxide portion 106. A regulator 108, which comprises a plurality of thick oxide devices, resides on the thick oxide portion 104, and a load circuit 110, which comprises a plurality of thin oxide devices, resides on the thin oxide portion 106. A battery 120 supplies power to the regulator 108 via a power-in pin 122. An output 130 from the regulator 108 is coupled to the load circuit 110 and provides a regulated voltage to the load circuit. The output 130 from the regulator 108 is also coupled to a power-out pin 132. An external capacitor 140 is coupled to the power-out pin 132 and to ground.
One known regulator 108 comprises an operational amplifier (not shown) and an analog drive device (not shown). Through a feedback loop, the operational amplifier maintains the voltage at the output 130 of the regulator 108 at the regulated level. At time of insertion of the battery 120, the analog drive device becomes active prior to the operation of the operational amplifier becomes established. As a result, the voltage of the output 130 of the regulator 108 can momentarily reach the voltage level of the battery 120, i.e., the output of the regulator overshoots the regulated voltage. The higher voltage level of the battery 120, while not high enough to damage circuits made from thick oxide devices, can damage circuits made from thin oxide devices.
Some ICs have a battery-save circuit. A battery-save circuit may be useful, during normal start-up to help prevent the voltage of the output of the regulator from going above the regulated level. This is because, during normal start-up, the digital circuitry that controls the battery-save analog transistor is always on because such digital circuitry does not turn-off during a normal turn-off procedure. However, such digital circuitry does turn-off as a result of removal of the battery, and, therefore, is not operating immediately prior to battery insertion. The battery-save circuit does not turn on fast enough to be effective at time of battery insertion. A typical battery-save circuit comprises digital circuitry whose output is coupled to a gate of an analog battery-save transistor. The analog drive device of the regulator turns on before the battery-save circuit turns on. Therefore, during battery insertion, the battery-save circuit cannot be relied upon to prevent the voltage of the output of the regulator to go above the regulated level.
If the load circuit 110 of the regulator 108 has a significant amount of capacitance, such as capacitor 140, the capacitance of the load can help absorb any voltage overshoot at the output 130 of the regulator that may occur immediately after battery insertion. Some ICs 101 save die area by not having a significant internal capacitance. If the load circuit 110 of the regulator 108 lacks a significant amount of capacitance, an excessive voltage overshoot during battery insertion is more likely to occur. When the load circuit 110 lacks sufficient capacitance to absorb a voltage overshoot, most known IC packages 102 use a power-out pin 132 for allowing a capacitor 140 external to the IC package to be coupled to the output 130 of the regulator 108.
Some IC packages save pin-out by not having the power-out pin. The outputs of regulators in such IC packages are more susceptible to voltage overshoots during battery insertion because such IC packages do not have any provision for allowing additional, external capacitance to be coupled to the output of such regulators.
Known protection circuits for ICs only address voltage overshoots that occur during normal start-up, that is, when the battery has remained continuously connected to the device during the period that the device was turned off.
Known protection circuits for ICs do not address protection from voltage overshoots that occur immediately after battery insertion. Known protection circuits for ICs do not address protection from voltage overshoots caused by the insertion of the battery 120.
Known protection circuits for ICs do not sense the supply voltage, such as the voltage of the battery 120, to limit the voltage of the output 130 of the regulator 108 to the regulated level.